* Split `helpers.py` into separate `url.py`, `permission.py` and `button.py`, dedicated to those separate concerns and update the docs accordingly
* Move `ThumbnailMixin` out to `mixins.py` and update documentation accordingly
* Ad #NOQA to import lines to hush pep errors
* Alphabetise helper import order
* - Delete `helpers/helpers.py`
- wagtal -> wagtail in docs
* `result_row_display` adds a `data-object_pk` attribute to each row, to make items easier to identify with JS
* Adds `get_extra_attrs_for_row()` method to `ModelAdmin`, to give developers a way of adding further attributes to the `<tr>` element
